282 SPECIFIC MICRO-ORGANISMS 



antitoxin and is designated as a toxic unit. The toxin is pre- 

 served in a dry state. To test a new antitoxin one employs 

 ToVo- of a toxic unit (40, ooo X the 15 + Ms dose) and ascertains 

 the amount of serum which must be added so as to neutralize it 

 to the L end point. Each trial mixture is diluted to i c.c. with 

 salt solution and 0.25 c.c. per 10 grams of body weight is injected 

 into a mouse. When the typical L effect is produced in the 

 mouse, the amount of antitoxic serum employed in the prepara- 

 tion of this particular mixture is said to represent ToW anti- 

 toxic unit. Ordinarily the mixtutre of toxin and antitoxin is 



A < -\ 



FIG. 115. Bacillus botulinus. Some individuals containing spores. (After van 



Ermengem.) 



allowed to stand 30 minutes before injection. Comparable re- 

 sults are obtained only by following a definite procedure and it 

 is especially necessary to use the conventional dose of roW 

 antitoxic unit and -nrVo toxic unit in the standardization of 

 sera. 



The standard unit employed in the United States is some- 

 what different from the Von Behring antitoxic unit. The Ameri- 

 can immunity unit of tetanus antitoxin is ten times the least 

 amount of antitetanic serum necessary to preserve the life of a 

 guinea-pig weighing 350 grams for 96 hours against the official 



